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Open circuit test
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Open circuit test
The Open circuit test, or "no-load test", is one of the methods used in electrical engineering to determine the no
load impedance in the excitation branch of a real transformer.
Method
The secondary of the transformer is
left open-circuited. A wattmeter is
connected to the primary. An ammeter
is connected in series with the primary
winding. A voltmeter is optional since
the applied voltage is same as the
voltmeter reading. Rated voltage is
applied at primary.
If the applied voltage is normal voltage
then normal flux will be set up. As the
Iron loss is a function of applied voltage, normal iron loss will occur. Hence the iron loss is maximum at rated
voltage. This maximum iron loss is measured using the wattmeter. Since the impedance of the series winding of the
transformer is very small compared to that of the excitation branch, all of the input voltage is dropped across the
excitation branch. Thus the wattmeter measures only the iron loss.
Since the secondary of the transformer is open, the primary draws only no load current. This no load current is
negligible. As the copper losses depend on current they can be neglected.
Current, voltage and power are measured at the primary winding to ascertain the admittance and power factor angle.
Another method of determining the series impedance of a real transformer is the short circuit test.
Calculations
The current
If
is very small.
is the wattmeter reading then,
The above equation can be rewritten as,
Thus,
Open circuit test
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Impedance
By using the above equations,
and
can be calculated as,
Thus,
or
Admittance
The admittance is the inverse of impedance. Therefore,
The conductance
can be calculated as,
Hence the susceptance,
or
Here,
is the wattmeter reading
is the applied rated voltage
is the no load current
is the magnetizing component of no load current
is the core loss component of no load current
is the exciting impedance
is the exciting admittance
References
• Kosow (2007). Electric Machinery and Transformers. Pearson Education India.
• Smarajit Ghosh (2004). Fundamentals of Electrical and Electronics Engineering. PHI Learning Pvt. Ltd..
• Wildi, Wildi Theodore (2007). Electrical Machines , Drives And Power Systems, 6th edtn.. Pearson.
Article Sources and Contributors
Article Sources and Contributors
Open circuit test Source: http://en.wikipedia.org/w/index.php?oldid=442216923 Contributors: After Midnight, Finog, Juzaf, Philip Trueman, PieterJanR, Ramajois, Sadads, So-called vita, The
Missing Piece, Theo10011, Thickycat, Toffile, Tyw7, Wtshymanski, Xoder, 14 anonymous edits
Image Sources, Licenses and Contributors
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License
Creative Commons Attribution-Share Alike 3.0 Unported
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